Inhomogeneous particles (so-called lump(s)) are mixed in a grease, especially in a diurea grease. The lump includes those supposedly derived from reaction products of isocyanate and amine, and foreign substances mixed during, for instance, a manufacturing process and/or storage. The lump herein also refers to both of those derived from the reaction products, and the foreign substances.
A typical manufacturing method of a urea grease includes: mixing a base oil with isocyanate to prepare a first solution, which is heated and stirred approximately at 60 degrees C.; mixing a base oil with amine to prepare a second solution kept at approximately 60 degrees C.; adding the second solution to the first solution while heating the first solution to approximately 60 degrees C. with stirring; heating the obtained mixture to approximately 160 degrees C.; and subsequently cooling the mixture to the room temperature. However, this method takes time in manufacturing (i.e., synthetic reaction) and is likely to generate the lump. Large lumps are known for deteriorating an acoustic property when the grease is used in a slide device such as a bearing. Further, since the thickener having a nonuniform structure formed of large lumps less contributes to an inherent performance of the grease, an efficiency of the thickener is decreased. In other words, a lot of thickener is required in order to obtain a predetermined hardness.
Accordingly, a grease manufacturing method for inhibiting formation of large lumps and improving an acoustic property has been proposed (see Patent Literatures 1 to 6).